John Gill Commentary Numbers 22:12

"And God said unto Balaam, Thou shalt not go with them; thou shalt not curse the people; for they are blessed." — Numbers 22:12 (ASV)

And God said to Balaam, you shall not go with them
Which is a denial of the first thing Balak requested, "come now, therefore" (Numbers 22:6)

you shall not curse the people ;
which was the principal thing desired, and in order to which he was solicited to go with the messengers; but this is absolutely forbidden:

for they are blessed ;
by the Lord himself, with an irrevocable blessing, and therefore it would be vain and fruitless, as well as dangerous for him to attempt to curse them, (Genesis 12:3) this may have a special respect to the blessing of Jacob by Isaac, which could not be reversed by the solicitations of Esau, and which descended to Jacob's posterity, the Israelites, (Genesis 27:33) .